Open-tip and Cl--selective microelectrodes were used to study the effect of external pH on apical membrane potential (Va) and intracellular chloride activity (aiCl) in epithelial cells of Necturus gallbladder. Increasing the pH from 7.2 to 8.2 in the mucosal, the serosal, or in both bathing solutions simultaneously, hyperpolarized Va (control value -60 +/- 5 mV) by about -6, -10 and -17 mV, respectively, but did not significantly change the transepithelial potential (VT = 0.3 +/- 0.5 mV). Identical hyperpolarizations were recorded with Cl--selective microelectrodes, even 40 min after changing external pH. Thus, aiCl (12 +/- 2 mM) remained essentially constant. The ratio fVa between the deflections in Va and VT produced by transepithelial current pulses, which is an approximate measure of the fractional resistance of the apical membrane, decreased when mucosal pH was increased, and increased when serosal pH was raised. The changes in Va and fVa are due, in part at least, to the known pH dependence of cell membrane K+ conductance (PK) in this tissue. The constancy of aiCl, despite significant increases in Va, indicates that cell membrane Cl- conductance (PCl) is virtually zero or decreases, with increased external pH, in a way that compensates for the increased driving force for Cl- exit. Experiments in which 90 mM gluconate or 90 mM methylsulfate were substituted for an equivalent amount of luminal Cl- did not provide any evidence for a significant contribution of Cl- ions, per se, to the emf or conductance of the apical membrane. They suggested, rather, a dependence of apical membrane cation permeability on luminal Cl- concentration. Since basolateral membrane PCl is known to be very low, the insensitivity of aiCl to Va is the consequence of a negligible electrodiffusive Cl- permeability at both cell membranes. Thus, overall, transcellular Cl- transport in Necturus gallbladder is, in large measure, effected by electroneutral processes.